Deposited thin films have major, diverse technological functions and enormous commercial value. There now exist diverse methods for vapor deposition of metals, semiconductors, insulators and organics, as well as complex multicomponents such as oxides and nitrides; these are described, for example, in R. Bunshah's text. In general, deposition methods fall into two classes. In Chemical Vapor Deposition (hereinafter, `CVD`), precursor molecules react at a heated substrate, usually at relatively high pressures, to generate species that comprise the deposited film. In Physical Vapor Deposition (hereinafter, `PVD`), the film species are generated some distance from the substrate, usually in a high vacuum; the gas phase mean free path is large, and film species travel by "line of sight" to deposit on the substrate.
Deposition techniques do not always fit this traditional description. The assignee of the present invention has developed a family of vapor deposition methods referred to as Jet Vapor Deposition (JVD), in which film species are made remotely, as in PVD, but the vacuum is "low", and the mean free path small. Film components travel "line of sight" because they are convected in a sonic, collimated, "jet in low vacuum". Exemplary processes, based on patented JVD sonic nozzle sources such as the hot filament wirefeed, and the "electron jet" or "e-jet", include deposition of metals such as Cu, Au, Ag, Sn, Pb, Ni, Ti, Ta, and many others, singly or as alloys, in simple or multilayer form. However, it is also possible in JVD to convect species to the substrate that then undergo film forming or film modifying reactions at the surface just as in CVD.
Polymer films are of interest for protective and electrical properties, and several methods exist for their deposition. A simple example is the "spin-on" of liquid precursors, as in the familiar first step of semiconductor photo-lithography. A more general and powerful approach is plasma polymerization. The effects of different plasma conditions are described in the book "Plasma Polymerization" by H. Yasuda, Academic Press, Inc. 1985, and the critical importance of free radicals is discussed in detail.
However, there are a few molecules which are exceptional in that the generation of film forming radicals can easily be accomplished by simple thermal means, without need of assistance from a plasma. The Parylenes are the outstanding example of this type, and their behavior is unusual. Heating of a simple precursor molecule creates radicals that then polymerize on a room temperature or cooled surface. The basic chemistry [M. J. Szwarc, Disc. Far. Soc. 2 46 (1957)] and commercial process [Gorham, U.S. Pat. No. 3,342,754, September 1967)] are now well known, although much still remains to be learned about the mechanisms of Parylene deposition. FIG. 1a shows the structure of the dimer Parylene precursor, di-para-xylylene, 1, and the subsequent steps needed to deposit poly-p-xylylene. The dimer 1 is vaporized at .about.150.degree. C. (not shown) and passed through a "cracking" section at .about.680.degree. C. where it is thermally split into two reactive monomers which can exist either in the di-radical form 2, with two unpaired electrons, or the quinonoid form 3. Both forms are highly reactive.
Deposition of these monomers occurs on surfaces held at room temperature or below; polymerization occurs in the film by a free radical mechanism which has been much studied but is still incompletely understood. The growing film is regarded as "quasi-liquid"; the monomer is first physically adsorbed on the surface, and then may diffuse into the growing film, where it can react with the ends of other radical chains. The deposition rate is determined and limited by the rate of adsorption and reaction of reactive monomer in the film. Cooling the substrate has a large effect on growth rate, by increasing the physical adsorption lifetime for depositing monomers thereon and hence the time available for reaction.
Parylene occurs in several modifications, denoted as Parylene N, C or F, as seen in FIG. 1b; the Cl and F substituents alter the properties of the film, and affect the ease of deposition. For example, Parylene C will deposit at room temperature, because the strong C-Cl dipoles promote the long physical adsorption lifetimes that favor polymerization. Parylene N, less polar, will not deposit on any surface above 30.degree. C., and useful deposition rates are only gotten on surfaces cooled much below room temperature. Parylene N has a lower dielectric constant, however, than Parylene C. Low dielectric constant is a desirable property, making the Parylenes top candidates for "low K" applications in integrated circuits.
Although some variations have appeared in the literature, systems for deposition of the Parylenes are based on the Gorham process described above. They normally operate in a range of low pressures, 20-80 millitorr. The gas flow speeds are very small; indeed, carrier gases need not be used at all. The systems are many feet long, extremely large by comparison with JVD systems, in which a typical source can be mounted on a 4" flange, and held in one hand.
Because reactive Parylene monomers are easily generated by heating, as was illustrated in FIG. 1a, it might appear to be of no advantage to allow the gas phase monomers or the growing film to interact, during deposition, with energetic species emerging from a plasma. Indeed, employing such direct interaction is heretofore unknown and is a major aspect of the present invention.